The Synthesis And Electrochemical Performance Characterization Of Si-carbon Composite Anode Materials For Lithium-ion Batteries

In recent years, lithium ion secondary battery is considered to be one of the most promising vehicle energy storage devices along with environmental pollution problems and the vigorous development of electric vehicles. At present, the researchers are working to improve the energy density and power density of lithium-ion batteries, to meet the development trend of energy. Graphite and modified graphite are mostly used as anode material in commercial lithium-ion batteries. Despite the advantages of graphite electrode, such as flat potentials as low as Li metal and high cycling stability at high rates, the graphite electrode can be polarized to such an extent that lithium dendrites/moss form on the surface of the negative electrode surface, potentially rendering thermal runaway because of internal shorts triggered. In addition, the theory capacity of graphite is only 372 mAh/g. This has spurred intense interest in developing high security, high gravimetric and volumetric capacity battery electrodes to surpass the energy density of the current generation of lithium batteries. Silicon has been demonstrated as a promising anode material for lithium-ion batteries [LIB], due to its high theoretical capacity of 4200 mAh/g, which is more than ten times higher than that of the graphitic carbon (372 mAh/g). However, one major problem preventing them from the commercial application is that they undergo large volume changes during cycling, which results in disintegration of the electrodes and subsequent rapid capacity fading.One of the effective strategies is to employ the nano-sized silicon and nanocomposite, especially the carbon medium supporting composite. Carbon can not only be used as a softmediumto buffer the stress of volume expansion, but also maintain the electrical connectivity between the Si particles. In order to improve the stability and uniformity of the silicon nanoparticles load on graphene, this thesis innovatively put forward the in situ growth method silicon nanoparticles on graphene. Secondly, this thesis also innovatively designs the three-dimensional multi-scale interconnected sandwich carbon/silicon/carbon nanospheres composite. This new type structure is also applicable to other alloy anode materials system. Based on the above research train of thought, the research content of this thesis is as follows:In Chapter 3, we mainly realized good dispersion of Si nanoparticles on graphene sheets. Graphene was used to improve the structure stability of silicon nanoparticles and increase the electrode conductivity because of varieties of advantages of graphene, such as high two-dimensional electrical conductivity, superior mechanical flexibility, high chemical and thermal stability and large surface area. The defect and some chemical function group existing on graphene sheets made the nano-sized SiO2 homogeneously load on graphene sheets. After magnesium thermal reduction of SiO2, 5 nm-Si nanoparticles/graphene composite was obtained, such small Si particles typically show small absolute volume changes during lithium-ion insertion/extraction. The nanospaces between the particles also provide a good volume buffer. Moreover, the good contact between Si nanoparticles and graphene obtained by the in-situ growthmaintains the electrical connectivity during Li-ion insertion/extraction. And compared with the common method, such as ultrasonic dispersion method, self-assembly method under electrostatic attraction, the high energy ball milling method and spray drying method, the in situ generated method is more facile and more suitable for uniform dispersion of nanostructured silicon, and this method can avoid nanoparticles reunion. This method also applies to other nanomaterials evenly dispersed on the graphene.Seeking new architecture is still one of the development approaches in improving the properties of silicon-carbon composites. The three dimensional structure and micro/nano multi-scale of electrode materials are not only useful for lithium ion transport, also, the thinner wall of holes is useful for lithium ion diffusion, thus the composite will present higher capacity and better rate performance. In Chapter 4, the template method was introduced into the synthesis process, we designed and synthesized a silicon-carbon composite with three dimensional structure. The structure can not only present the advantages of micron scale to achieve the subsequent homogeneous carbon coated and then improve electrical conductivity; but also the nanoscale structure can reduce the volume changes and shorten the transmission path of lithium ion. The three-dimensional structure is the interconnected sandwich structure, nano-silicon is surrounded with carbon, and thus, carbon coated along with the interconnected spherical structure like a giant conductive grid, its electrical conductivity is obviously improved. In addition, the unreacted silica remained in the main body innovatively can not only be used as an inert medium to buffer volume expansion, also can support the structure and provide good skeleton structure, eventually improve the cycle stability of the electrode. There is a close relation between the excellent stability and three-dimensional multi-scale structure, this design method is also applicable to other alloy anodes materials with volume expansion.